Java - Fibonacci Numbers

Question

Any idea why my code is giving out the wrong answer after the first few?

My university professor provided us with this to go by and I feel I followed it?

If for example I was to use:

System.out.println(fibr(8));
System.out.println(fibr(9));
System.out.println(fibr(10));

The console prints out: 11 34 20

Which of course is not the fibonacci numbers in those places

public static int fibr(int n) {
    if(n<0) return 0;
    if(n==0) return 0;
    if(n==1) return 1;
    if(n==2) return 1;

    //is odd
    // n is = or > 3 and NOT (n divided by 2 with remainder of 0 (making it even))
    if(n >= 3 && !(n % 2 == 0)) {
        int a;
        a = fibr((n+1)/2) * fibr((n+1)/2);
        a = a + (fibr((n-1)/2) * fibr((n-1)/2));
        return a;
    }

    //is even
    if(n >= 3 && (n % 2 == 0)) {
        int a;
        a = fibr((n/2)+1) + fibr((n/2)-1) * fibr(n/2);
        return a;
    }
    return 0;
}

Help what's wrong cri

Answer 1

If your professor wants you to use the provided formula, then you should rewrite your code like this:

public class FibClass {

    // main
     public static void main(String[] args) {

      System.out.println(fibr(7)); // Returns 13

     }


    // Fibonacci function
     public static int fibr(int n) {

          if ( n < 0 ) {
            return 0; //returned value for negative integers
          } else if (n == 0) {
            return 0; //returned value for 0
          } else if (n == 1) {
            return 1; //1st number in the Fibonacci sequence
          } else if (n == 2) {
            return 1; //2nd number in the Fibonacci sequence
          }


          //is odd
          else if (n >= 3 && !(n % 2 == 0)) {

           int a;
           a = (int) Math.pow(fibr((n + 1) / 2), 2);
           a += (int) Math.pow(fibr((n - 1) / 2), 2);
           return a;

          }

          //is even
          else if (n >= 3 && (n % 2 == 0)) {

           int a;
           a = (fibr((n / 2) + 1) + fibr((n / 2) - 1)) * fibr(n / 2);
           return a;

          }

      return 0;

     } // END Fibonacci function fibr()


}

This is based on the formula you provided in the photo:

Although there are more simple functions to find the n-th number in the Fibonacci sequence.

Answer 2

Your code is pretty difficult to read which, in practice, makes debugging very difficult. Try to take a simpler approach. Here is an example to find the n-th Fibonacci number:

double fibbonaci(int n){
    double prev = 0d, next = 1d, result = 0d;
    for (int i = 0; i < n; i++) {
        result = prev + next;
        prev = next;
        next = result;
    }
    return result;
} 

Additionally, there is a more elegant method to this problem which involves recursion:

int fibonacci(int n)  {
    if (n == 0)
        return 0;
    else if (n == 1)
        return 1;
    else
        return fibonacci(n - 1) + fibonacci(n - 2);
}

Edit: To fully answer your question, your error is in calculating even Fibonacci numbers. fib(8) = 21, fib(9) = 34, fib(10) = 55. For some unknown reason, you are calculating the n-th even Fibonacci number using:

int a;
a = fibr((n/2)+1) + fibr((n/2)-1) * fibr(n/2);
return a;

By the Fibonacci sequence, the n-th Fibonacci number is the sum of the previous two. So the above code can become:

return fibr(n - 1) + fibr(n - 2);

Therefore, your entire function can be reduced to:

public static int fibr(int n) {

    if (n <= 0) return 0;
    if (n == 1) return 1;
    if (n == 2) return 1;

    return fibr(n - 1) + fibr(n - 2);
}